Fix R128_TRACK_GAIN tags for flac input.

opusenc was emitting an extra '=' character.
Thanks to Jan Janssen for the report.
Add support for METADATA_BLOCK_PICTURE.

The allows embedding album art in Opus files.
The format is the same as for Vorbis files, which is a base64
 encoded VorbisComment version of the format used in FLAC files.
It extends opusenc to provide a --picture option that works the
 same as FLAC's, and automatically parses mime type and image
 dimensions for JPEG, PNG, and GIF files.
It also updates the FLAC input module to convert FLAC's native
 metadata picture blocks to the base64 encoded version.
Add METADATA_BLOCK_PICTURE validation to opusinfo.

This will parse such comments and validate the various pieces, and
 print the results in a form similar to that used by the --picture
 argument to opusenc.
Refactor METADATA_BLOCK_PICTURE support.

This splits some common parts into a separate picture.h and
 picture.c.
It also allows opusinfo to verify the declared
 width/height/depth/colors information against the values in the
 actual image data (if found).
Add picture.c to Makefile.unix.